1. Field of the Invention
The present invention relates to a photographic optical system and an image pickup apparatus that can use the photographic optical system. More specifically, the present invention relates to a photographic optical system useful in an image pickup apparatus, such as a video camera, a digital still camera, a television (TV) camera, or a monitoring camera, and to an image pickup apparatus including the photographic optical system.
2. Description of the Related Art
A conventional telephoto type photographic optical system having along focal length includes, in order from the object side to the image side, a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power. Generally, in a photographic lens having a long focal length (telephoto lens), chromatic aberration tends to occur. Specifically, in a telephoto lens, the longer the focal length becomes, the more the amount of chromatic aberration, such as axial chromatic aberration and chromatic aberration of magnification, may occur.
A conventional telephoto lens corrects the above-described chromatic aberration by using a positive lens made of a low dispersion material having anomalous partial dispersion, such as fluorite or FK01 (a product of OHARA INC.) and a negative lens made of a high dispersion material in combination with each other. U.S. Pat. No. 5,757,555 discusses a telephoto lens having a focal length of 294 to 392 mm and an F-number of 4.08 to 5.6.
In addition, a large amount of spherical aberration and coma may occur in a telephoto lens having a small F-number. In order to correct spherical aberration and coma occurring on a telephoto lens having a small F-number, a conventional technique is to increase the flexibility of correcting aberrations by increasing the number of lenses. U.S. Pat. No. 5,745,306 discusses a telephoto lens having a high aperture ratio, whose focal length is 294 to 588 mm and F-number is 2.88 to 4.08, which corrects aberration by using an increased number of lenses.
On the other hand, a conventional photographic optical system uses a diffractive optical element to reduce the total weight of the photographic lens while various aberrations including chromatic aberration that may occur in the optical system are corrected at the same time. By using a diffractive optical element, the weight of a photographic optical system can be reduced while correcting chromatic aberration at the same time by reducing the lens total length and by using a lens made of a glass material whose specific gravity is relatively low.
U.S. Pat. No. 7,426,083 discusses a telephoto lens having a high aperture ratio capable of appropriately correcting chromatic aberration by using a diffractive optical element. More specifically, the telephoto lens discussed in U.S. Pat. No. 7,426,083 has a focal length of 493 to 780 mm and an F-number of 4.1 to 5.8.
The telephoto lens discussed in U.S. Pat. No. 7,426,083 uses an optical system having a short lens total length and uses a positive lens included in a front lens unit made of a glass material whose specific gravity is relatively low. Accordingly, the above-described conventional telephoto lens can reduce the total weight of the optical system while correcting chromatic aberration.
In most conventional photographic optical systems, focusing from an infinitely-distant object to a short-distance object is executed by moving the entire photographic lens system or a part of lens units included in the photographic lens system. The size of a telephoto lens having a long focal length may be large. As a result, the weight of a conventional telephoto lens tends to be rather heavy. Accordingly, if the telephoto lens like this is used, it becomes difficult to execute focusing by moving the entire telephoto lens system.
Therefore, most conventional telephoto lens systems execute focusing by moving a specific lens unit(s) among those included in the telephoto lens system. As one type of the photographic optical systems described above, various inner-focus type photographic optical systems have been used. In an inner-focus type photographic optical system, a relatively small and lightweight lens unit, which is provided at the center of the optical system, is moved during focusing.
Each of telephoto lenses discussed in U.S. Pat. No. 5,757,555, U.S. Pat. No. 5,745,306, and U.S. Pat. No. 7,426,083 includes, in order from the object side to the image side, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power. Furthermore, each of the conventional telephoto lenses described above employs an inner-focus method, by which focusing is executed by moving the second lens unit along the optical axis towards the image side.
In a telephoto lens, generally, the longer the focal length becomes, the greater in size and the heavier in weight a front lens unit having a positive refractive power becomes. Accordingly, in order to achieve a high optical performance by appropriately correcting chromatic aberration while reducing the total size of the photographic optical system, it is significant to appropriately set the lens configuration of the front lens unit having a positive refractive power.